1. Field of the Invention
The present invention relates to a reversible thermosensitive recording medium which utilizes the coloring reaction between an electron donor coloring compound and an electron acceptor compound capable of inducing color formation in the electron donor coloring compound.
2. Discussion of Background
Conventionally there have been proposed various reversible-color-changeable materials with application of heat thereto, such as metal complex salts, cholesteric liquid crystals, and mixed systems composed of an electron donor coloring compound (hereinafter referred to as a coloring agent), an electron acceptor compound (hereinafter referred to as a color developer) and a coloring and decolorization temperature controlling agent.
These reversibly-color-changeable materials are widely used, because of the reversibly color-changeable characteristics thereof, in a variety of fields, such as in the fields of temperature-measurement materials for measurement or control of temperature, various display devices, clothes, inks, toner, writing instruments, teaching materials, and toys.
In particular, reversibly color-changeable materials, which are composed of a coloring agent, a color developer and a color and decolorization temperature controlling agent as indispensible components, have been proposed for use in various fields because of the advantages thereof over other materials that a wide color-changeable temperature range can be set, sharp and large color changes can be obtained, a great variety of colors can be obtained, and such reversibly color-changeable materials are innoxious, as disclosed, for instance, in Japanese Patent Publications 51-44706, 51-44709, 52-7764, Japanese Laid-Open Patent Applications 63-15877, 63-251487, 1-121394 and 1-174591.
Most of the reversibly color-changeable materials composed of a coloring agent, a color developer and a coloring and decolorization temperature controlling agent are of a thermal decolorization type, in which a colored state obtained by the interaction between the coloring agent and the color developer is decolorized to a colorless state by the application of heat thereto.
Furthermore, there have been proposed reversibly color-changeable materials of a thermal color-development type, in which a colored state is reversibly produced from a colorless state by the application of heat thereto.
An example of a reversibly color-changeable material of such a thermal color-development type is proposed in Japanese Laid-Open Patent Application 4-14482. In this Application, a thermosensitive recording layer comprising a leuco dye, a color developer and a decolorization promoting agent is provided on a substrate, and the recording layer is colored with application of heat thereto, for example, by a word processor, and the developed color is then allowed to be decolorized with time.
In this reversibly color-changeable material, the recording density and erasability thereof are lowered in the course of the repetition of the color formation for recording and the erasure thereof.
The Applicants of the present invention have previously developed a reversible thermosensitive recording medium having a recording layer comprising a coloring agent and a color developer, which is capable of forming a stable colored recording state at room temperature by heating the coloring agent and the color developer to a color development temperature to fuse the coloring agent and the color developer, and also capable of changing the colored recording state to a decolorized state by heating the same to a temperature lower than the color development temperature, and maintaining the decolorized state stably at room temperature as disclosed, for instance, in Japanese Laid-Open Patent Application 5-124360. The erasability of images recorded in this reversible thermosensitive recording medium, however, cannot reach a satisfactory level when recording and erasure of images are repeated a number of times.
Furthermore, the Applicants of the present invention have proposed a reversible thermosensitive recording medium in Japanese Laid-Open Patent Application 5-69664, which is prepared by coating microcapsules containing a coloring agent, a color developer, a coloring and decolorization temperature controlling agent and a solvent with low volatility together with a binder resin on a substrate. The thus prepared reversible thermosensitive recording medium is excellent in (a) the thermal coloring performance from a colorless state to a colored state, (b) the erasability at room temperature, and (c) the repeated use characteristics thereof. However, the necessity for the formation of the microcapsules makes the production of this reversible thermosensitive recording medium difficult.
In addition, the Applicants of the present invention have discovered the following preferable decolorization promoting agents (1) to (8) for use in a reversible thermosensitive coloring composition comprising a coloring agent, a color developer and a decolorization promoting agent, as disclosed in Japanese Laid-Open Patent Application 5-294063:
(1) Fatty acids, and derivatives and metal salts of fatty acids; (2) waxes, fats and oils; (3) higher alcohols; (4) phosphoric esters, benzoic esters, phthalic esters, and hydroxy acid esters; (5) silicone oil; (6) liquid crystalline compounds; (7) surfactants; and (8) organic compounds having saturated hydrocarbon chains with 10 or more carbon atoms.
The Applicants of the present invention have discovered that in a reversible thermosensitive recording medium which comprises a substrate and a recording layer formed thereon comprising a coloring agent, a color developer and a decolorization promoting agent, a color formation step of forming a recording state with application of heat and a decolorization step of allowing the recorded state to stand under normal conditions can be repeatedly carried out by use of at least one material selected from the group consisting of higher alcohols, higher esters and higher amides as the decolorization promoting agent, as disclosed in Japanese Laid-Open Patent Application 6-155905.
The decolorization promoting agents disclosed in the above Japanese Laid-Open Patent Applications 5-294063 and 6-155905, however, are still insufficient for obtaining practically usable repeated use performance.
Thus, conventional reversible thermosensitive recording media utilizing the coloring reaction between a coloring agent and a color developer have various problems and are unsatisfactory for use in practice.